1. Field of the Invention
An aspect of this disclosure is related to a technology for delivering positional information.
2. Description of the Related Art
There exists technology for determining positions of communication terminals of users using a global positioning system (GPS). In the GPS, about 30 GPS satellites orbiting the Earth transmit radio signals indicating time. A communication terminal on the Earth including a GPS receiver receives radio signals from some of the GPS satellites and calculates the distance between the communication terminal and the GPS satellites based on differences between the time the radio signals are transmitted from the GPS satellites and the time the radio signals are received at the communication terminal. Generally, the communication terminal calculates the distances between the communication terminal and at least four GPS satellites, and determines the position of the communication terminal on the Earth based on the calculated distances.
As the size and power consumption of GPS receivers are reduced by recent technological developments, GPS receivers are installed even in small communication terminals such as cell phones that are driven by a battery.
One problem with the GPS is that GPS radio signals can hardly reach communication terminals located in an indoor space (or area). For this reason, a system different from the GPS is desired to measure positions of communication terminals located in an indoor space. As an example of such a system, an indoor messaging system (IMES) is now under the spotlight.
A broadcasting device according to the IMES can broadcast a radio signal having the same radio wave format as the radio signal transmitted by GPS satellites. Therefore, a communication terminal can receive the radio signal from the broadcasting device using the same hardware as that used to receive the radio signal from GPS satellites with only minor changes made to reception software. The radio signal broadcast by the broadcasting device includes, instead of time information indicating time, positional information indicating the position of the broadcasting device. Accordingly, a communication terminal receiving the radio signal does not need to calculate the time differences to determine its position as in the outdoors.
Also, Japanese Laid-Open Patent Publication No. 2011-145873 discloses a position management method employing the IMES. In the related-art method disclosed therein, a communication terminal receives positional information from an IMES broadcasting device installed on an indoor ceiling and transmits the positional information and a terminal ID of the communication terminal to an access point of a wireless LAN (local area network) according to a communication standard defined as IEEE802.11x; the access point transfers the positional information and the terminal ID to a management server; and the management server manages the position of the communication terminal based on the positional information and the terminal ID.
With the related-art method, however, the communication terminal needs to have a wireless LAN transmitter supporting the communication standard of the wireless LAN to transmit the positional information and the terminal ID. Also, compared with a recent GPS (or IMES) receiver with reduced power consumption, a wireless LAN transmitter consumes a large amount of power. Accordingly, with the related-art method, even if the power consumption of the receiver of a communication terminal is reduced, it is difficult to reduce the total power consumption of the communication terminal due to the large power consumption of the transmitter.